The present invention relates to a method and apparatus for monitoring and/or quantitatively measuring the respiration of a patient and, particularly, to a method and apparatus for monitoring a patient""s respiration using a flexible piezoelectric film sensor.
Electro-mechanical devices may be used to measure the physiological characteristics of a subject for monitoring particular physical characteristics and diagnosing the subject""s health. Respiration monitors which seek to quantify a patient""s breathing are one category of patient monitoring device. In conventional hospital respiration monitors, a patient""s breathing is monitored by a technique known as impedance pneumography. Electrodes are placed on the patient""s chest and trans-thoracic electrical impedance is measured with a small high-frequency current. As the patient breaths, air fills the lungs resulting in a change to the chest""s electrical impedance. An impedance pneumograph measures this impedance variation/modulation in order to determine the respiratory status of the patient. This technique, while widely used in clinical environments, suffers from several major limitations. First, the technique is fundamentally a measure of impedance changes related to chest motion. Therefore any patient motion, whether due to respiration or other movements, will cause a modulation in the impedance signal. Such non-respiratory modulations are referred to as artifacts. These artifacts often contribute to erroneous calculation of respiration rate, false alarms, and undetected cessation of breathing (also known as apnea).
Second, the impedance pneumography technique monitors respiratory effort, but does not measure actual respiratory airflow. In certain patients, a blockage of the airway may restrict the amount of air entering the lungs (obstructive apnea), but because the chest wall continues to move (as the patient attempts to breathe) impedance pneumograph respiratory monitors will not indicate any abnormal breathing patterns. The inability of impedance pneumography to detect obstructive apnea is a severe limitation of the technique in a clinical setting.
Alternatively, respiration can be monitored through a device using one or more thermistors. Thermistors are resistors made of semiconductors having resistance that varies rapidly and predictably with temperature. The thermistors detect respiration by measuring temperature changes in air temperature directly adjacent to a patient""s airflow. Normally such devices include portions which cover one or more nostril or the patient""s mouth. As such, thermistors can impede a patient""s natural breathing and do not accurately measure respiration.
Other existing respiration monitors directly measure inspired and expired air through measurement of air flow using one or more air flow transducers. In such existing patient monitors, bi-directional air flow passing one end of a tube induces unidirectional air flow at the other end which is detected by a transducer to provide output signals corresponding to the movement of air. Like thermistors, however, such an arrangement can impede the patient""s breathing and is not adaptable for use as a diagnostic tool for respiratory conditions.
Accordingly, there is a need for improved methods and apparatus for monitoring a patient""s respiration that quantitatively measure respiration, are resistant to artifact interference, suitable for use as a diagnostic tool and which do not suffer from the drawbacks associated with existing respiration monitors.
The present invention provides a method and apparatus for monitoring a patient""s respiration which uses a sensor placed on the surface of a patient""s neck to measure sound waves generated by respiratory airflow to monitor and/or quantitatively measure the respiration of a subject. Specifically, the present invention utilizes piezoelectric film (piezo film) placed on a patient""s neck as a respiratory sensor.
The invention is manufactured using a piezoelectric film (utilizing the piezoelectric effect) arranged and configured to perceive acoustic waves associated with respiration while eliminating non-respiratory sound artifacts. Voltage is produced between opposite surfaces of the piezo film when mechanical forces are applied. As such, the piezoelectric effect converts mechanical acoustical energy into electrical energy. Piezo film has several beneficial characteristics when used as a sensing element. First, piezo film is thin, flexible and light, providing a comfortable, yet tightly coupled interface to the patient which does not inhibit natural respiration. Second, respiration monitors based on the present invention are relatively inexpensive, allowing the sensor to be disposable or semi-disposable. Third, piezo film sensors are passive devices, which allow the present invention to function without subjecting the patient to electrical excitation. Finally, the sensor""s acoustic impedance is well matched to tissue, allowing maximum signal transfer from the skin to the sensor while simultaneously rejecting external acoustic vibrations that reach the sensor through the air.
Preferably, the sensor is coupled to a device such as a patient monitor which acts as a signal acquisition unit accepting signals generated by the sensor, processes the signals and correlates the signals over time to determine one or more physiological characteristics associated with respiration.
An advantage of the method and apparatus described is that respiration of a patient may be monitored using an inexpensive sensor which is comfortable for the patient and suitable for monitoring or quantitatively measuring a wide range of respiratory characteristics.
An additional advantage of the present invention is that the device may be used to diagnose various respiratory conditions. Information contained in breath sounds may be used in new monitoring applications such as correlating the signature of the sound produced by a patient""s respiration to detect the presence of fluid collection in the lungs or detecting respiratory failure. As such, the present invention also provides a method and apparatus for diagnosing medical conditions associated with respiration, including, but not limited to apnea, the collection of fluid in a patient""s lungs and respiratory failure. Preferably, the sensor is coupled to a device which accepts signals generated by the sensor and analyzes the signals in comparison to respiratory auditory characteristics associated with known respiratory conditions.
Yet another advantage of the invention is that the present invention could be configured and adapted for use in other human air or fluid passage-ways, such as arteries or veins or, alternatively, on mechanical devices with air or fluid flow such as mechanical conduits or pipelines.
Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.